Polychlorinated biphenyls (PCBs) are organic chemicals that were produced on a large scale in the period 1930-1980. Approximately 600,000 tons of the material were manufactured and were used in a wide variety of applications. However, the most important use for the material was as an insulator in electrical transformers and capacitors. PCBs were ideally suited to this role because of their chemical inertness and lack of flammability.
In the mid 1970s concerns began to be expressed about the detrimental effects of PCBs on health and the environments. While these concerns were not substantiated to a large extent, there still existed a very strong public pressure to eliminate the use of PCBs. Accordingly, large scale manufacture was halted and attempts began to be made to eliminate PCBs from the environment. However, the very properties that made PCBs desirable in the first place--chemical inertness and lack of flammability--made their destruction extremely difficult.
The most common method for disposal is to dilute PCBs with combustible organic materials and to incinerate them at extremely high temperatures (1100.degree. C.).
This method has the significant drawback that incomplete combustion can lead to polychlorinated dibenzofurans which are known to be extremely toxic.
Many other methods have been developed most of which are based on the use of alkali metals (or their hydroxides), especially dispersions of sodium metal. A typical process using sodium is described in U.S. Pat. No. 4,340,471, Jul. 10, 1982. PCB-contaminated silicone-based or hydrocarbon oils have been treated with hydrocarbon dispersions of sodium (U.S. Pat. No. 4,379,746, Apr. 12, 1983). Sodium metal also has been used in the presence of an electron carrier (e.g., benzophenone, alkylbiphenyl) and an aprotic complexing solvent (e.g. tetrahydrofuran, dimethylformamide) in U.S. Pat. No. 4,377,471, Mar. 22, 1983. Japanese Patent No. 49082570 mentions the use of isopropanol with sodium and removes excess sodium with methanol. Carbon dioxide gas and water have been used to remove excess sodium (U.S. Pat. No. 4,416,767, Nov. 22, 1983).
Dehalogenation also has been carried out with alkali metal aromatic radical anion reagents e.g. sodium naphthalide, lithium anthracide--see U.S. Pat. No. 4,284,516, Aug. 18, 1981. This type of reagent has been used in the presence of ether-type solvents (U.S. Pat. No. 4,326,090, Apr. 20, 1982). The reaction may be quenched using carbon dioxide (U.S. Pat. No. 4,447,667, May 8, 1984).
Another type of dehalogenation has involved the use of hydrogen gas under pressure in the presence of a catalyst: the process requires elaborate equipment and is sensitive to impurities (U.S. Pat. No. 4,623,448, Nov. 18, 1986). Still another type of process has involved reaction with sulfur at high temperatures (U.S. Pat. No. 4,581,442, Apr. 8, 1986).
The PCB--contaminated silicone oils mentioned in prior art such as U.S. Pat. No. 4,379,746 are transformer oils, heat transfer fluids or lubricants based on polysilanes, and are distinct from hydrosiloxanes.
It would be desirable to provide such a dehalogenation process that would be more effective at room temperature, use relatively inexpensive reagents and equipment, and be relatively insensitive to impurities.